1. Technical Field of the Invention
The present invention relates to a semiconductor device including a semiconductor acceleration sensor which converts displacement into an electrical signal utilizing the piezoresistance effect of a semiconductor crystal such as silicon.
2. Description of the Prior Art
FIG. 4 shows a semiconductor acceleration sensor produced by micromachining that is disclosed in Japanese Unexamined Patent Publication No. Hei. 1-302167. A cantilever 21 is formed with a groove 23 in the vicinity of a support 24 by etching, so that a thin portion 22 is formed there. Piezoresistors 2 formed on the top surface of the sensor constitute a bridge circuit.
A weight 4 is provided at a tip portion of the cantilever.
As shown in FIGS. 10, Japanese Examined Utility Model Publication No. Hei. 3-20780 discloses an example in which a metal weight is formed by electroplating.
Further, FIG. 15 shows a semiconductor acceleration sensor disclosed in Japanese Unexamined Patent Publication No. Hei. 5-312827. A cantilever sensor element 1 is formed with a groove 23 in the vicinity of a support 24 by etching, so that a thin portion 22 is formed. Piezoresistors 2a, 2b, 2c, and 2d formed on the top surface of the sensor constitute a bridge circuit. The piezoresistors 2b and 2d which serve as reference resistors are disposed on the support 24. The piezoresistors 2a and 2c which serve as variable resistors for detecting the deformation amount of the thin portion 22 are so arranged as to be perpendicular to the reference resistors.
When the sensor element 1 of the cantilever receives acceleration, it is displaced, so that the resistance values of the piezoresistors 2a and 2c increase. Differences between the resistances of the piezoresistors 2a and 2c and those of the reference resistors 2b and 2d cause a voltage corresponding to the acceleration. This voltage is supplied to a semiconductor integrated circuit substrate 110 that is provided externally via wires 116, which performs signal processing such as amplification of a very small voltage and temperature compensation.
Further, FIG. 16 shows an example disclosed in Japanese Unexamined Patent Publication No. Hei. 6-331647 in which a cantilever sensor element 1 is formed by micromachining. In this example, signal processing circuits such as a detection circuit and an EEPROM are formed on a rim 113 that surrounds the cantilever of the sensor element 1, to compensate for the output characteristics of the sensor.
In the conventional semiconductor acceleration sensors, a set of piezoresistors for detecting acceleration which are provided on the surface constitute a bridge circuit as shown in FIG. 4.
In the acceleration sensor, the sensitivities in directions (other-axes directions) other than the detection axis need to be reduced. In the typical sensor configurations described above, no measures are taken to reduce the sensitivities in the other-axes directions.
The sensitivities in the other-axes directions result from an inclination of the sensor element which is introduced when the sensor element is fixed to a base, a deviation between the centers of gravity of the weight and the sensor element, and other factors.
In the case of Japanese Unexamined Utility Model Publication No. Sho. 61-102871 (see FIG. 5), weights 4 are provided on both top and bottom surfaces of a tip portion of a cantilever 21 of a sensor element so that the center of gravity of the two weights as a whole coincides with the center of the cantilever in the thickness direction. This indicates how important the positional relationship between the centers of gravity of the sensor element 1 and the weights 4 is.
In the methods of adding the weight 4, the weight 4 needs to be disposed at the center of gravity of the sensor element having the piezoresistors. However, a positional deviation of the weight 4 deteriorates the characteristics because torsional stress on the sensor element is increased and acceleration comes to be detected in directions other than the detection axis. This is other-axes sensitivity characteristic of the acceleration sensor. The acceleration detection in directions other than the detection axis is a cause of erroneous operation.
As described above, as far as the characteristics of the acceleration sensor are concerned, it is necessary that the sensor element should not be influenced by torsion and the like.
In addition, in the conventional acceleration sensors, the weight is attached to an end portion of the sensor element with an adhesive or the like. This causes major problems that the number of manufacturing steps increases and that there may occur a positional deviation between the centers of gravity of the sensor element and the weight. It is important to decrease a positional deviation of the weight from the sensor element.
Turning to another aspect of the semiconductor acceleration sensor as shown in FIG. 15, in order, to increase the functionality of the sensor, it is necessary to connect the semiconductor integrated circuit substrate 110 for signal processing to the output stage of the sensor.
In general, an output voltage from the acceleration sensor which is caused by displacement due to acceleration is several millivolts. Therefore, connection to an amplifier circuit 111 is highly necessary. Further, since the strain sensing portion consists of the piezoresistors, a temperature variation occurs. This necessitates a temperature compensation circuit.
Conventionally, the sensor element 1 and the semiconductor integrated circuit substrate 110 for signal processing are disposed adjacent to each other and connected together by wire bonding using wires 116. In this case, because a very small signal is picked up, there is a possibility that noise is superimposed on the signal through the wires 116.
An object of the invention is to reduce this type of noise. Further, the invention is intended to lower the cost by making the material of the conventionally used support 24 unnecessary by using the semiconductor integrated circuit substrate 110 as the support 24.
Japanese Unexamined Patent Publication No. Hei. 4-6471 discloses an example in which a voltage variation occurring in piezoresistors of a cantilever sensor element 1 due to acceleration is supplied to a semiconductor integrated circuit substrate 110 as a weight provided at the other end and incorporating an amplifier circuit 111, and signal processing is performed in the semiconductor integrated circuit substrate 110. The cantilever sensor element 1 and the semiconductor integrated circuit substrate 110 are connected to each other via bumps 3. An outer lead 130 is electrically connected to the cantilever sensor element 1 with a wire 116. This example is shown in FIG. 17. In this method, since the semiconductor integrated circuit substrate 110 deals with very small, high-frequency signals, noise is likely to occur.
Further, as shown in the top view of FIG. 16, Japanese Unexamined Patent Publication No. Hei. 6-331647 discloses the example in which the signal processing circuit 112 is formed on the periphery of the sensor element 1. In this case, the cantilever as the sensor element 1 is formed by etching while the semiconductor integrated circuit on the periphery is protected from an etching liquid. This results in an increase in the number of manufacturing steps, and is therefore an obstruction to cost reduction. There is an additional problem of a reduction in yield.
The inventor has been made to solve the above problems. An object of the invention is therefore to reduce noise and attain high performance by directly joining together the sensor element 1 and the semiconductor integrated circuit substrate 110 for signal processing without using the wires 116 or the like, as well as to lower the cost by using the semiconductor integrated circuit substrate 110 as the support 24.